Alkaline pulping processes and especially kraft pulping are dominant in the production of cellulose, because alkaline pulping provides pulp fibers which are stronger than those from any other commercial pulping process. A well-known method for cooking wood chips is the batch process. In a conventional kraft batch process, wood chips are fed to the digester from bins, directly or by conveyor systems, and cooking liquor is added. The cooking liquor includes fresh cooking liquor containing a water solution of sodium hydroxide and sulfur compounds, normally referred to as white liquor, and spent liquor from previous cooks (black liquor) to cover the chips and control the liquor-to-wood ratio. When chips and liquor have been added, the cook is started by introduction of heat either indirectly or directly by steam. The cook itself consists of a heating period and an “at pressure” period. The cooking conditions are usually about 160-180° C., with a pressure equivalent to the corresponding boiling point. At the conclusion of the cook when the delignification has proceeded to the desired reaction degree, a blow valve in the digester is opened and the contents of the digester are discharged into a blow tank, as the hot liquor in the digester flashing into steam and forces the cooked pulp out of the digester.
During the cooking cycle the digester is continuously vented to remove air and other non-condensable gases from the system. Turpentine, steam and other volatile compounds are also released during this venting or gas-off period. If the digester has been heated and vented properly, most of the turpentine will come over by the time the cooking temperature and pressure has been reached (Drew, D. et al., Sulfate Turpentine Recovery, Pulp Chemicals Association, New York, 1971, p. 70). The vapors from the digester go to a separator, where black liquor and/or pulp that have been carried over is separated, and the turpentine, steam and non-condensable gases go to one or more condensers. The condensate, consisting of turpentine and water, goes to a decanter where the two separate. The turpentine overflow goes to the turpentine storage tank. The turpentine recovery of batch digesters is extensively described in the chapter “Turpentine Recovery from Batch Digesters” in the book Sulfate Turpentine Recovery by Drew, D. et al., Pulp Chemicals Association, New York, 1971, p. 65-93.
However, the above-mentioned conventional batch process is energy inefficient and produces pulp of low strength delivery.
Batch processes have therefore been developed for the purposes of, among others, saving energy. From the early 1980's, new emerging efficient kraft batch processes using various kinds of displacements started to gain ground. Characteristic for the liquor displacement batch processes is the recovery of hot black liquor at the end of cooking and reuse of its energy in subsequent batches. Good examples of this development are processes described in, e.g., Fagerlund, U.S. Pat. No. 5,578,149 and Östman, U.S. Pat. No. 4,764,251. The displaced liquors of usually over 100° C. are stored in one or several pressurized accumulators which usually contain a continuous heat recovery system (see, e.g. U.S. Pat. No. 6,643,410). As a result, the energy efficiency of batch cooking has increased.
The quality of the pulp was also improved by the liquor displacement batch method by avoiding digester discharge which utilizes hard hot blow techniques. Gentle digester discharge is typically accomplished by cooling the digester prior to discharge, relieving the overpressure in the digester and then pumping the cooked material from the digester (see, e.g., U.S. Pat. No. 4,814,042). Further development of liquor-displacement kraft batch cooking has also involved the combination of energy efficiency and efficient usage of residual and fresh cooking chemicals to achieve facilitated delignification and high pulp strength (see, e.g., U.S. Pat. Nos. 5,183,535 and 6,643,410). This can be accomplished by arranging the displacement at the end of the cook to first recover the “mother” black liquor, hot and rich in residual sulfur, in one accumulator and then to recover the portion of black liquor contaminated by wash filtrate and lower in solids and temperature in another accumulator. The accumulated black liquors are then reused in reverse order to impregnate and react with, respectively, the next batch of wood chips prior to finalization of the cook with hot white liquor. By this means it is has become possible to start a kraft cook with a high charge of sulfur and a low charge of hydroxyl ion and thus carry out important sulfur-lignin reactions in the hot black liquor pretreatment phase.
In liquor-displacement batch processes, the chips are normally totally covered by liquor. Typically, higher liquor-to-wood ratios are used compared to conventional batch cooking as higher liquor-to-wood ratio enables liquor displacements and more efficient liquor circulations. Moreover, the higher liquor-to-wood and the displacement procedure results in more even distribution of chemicals and heat throughout the contents of the digester. As a result, the produced pulp is more uniform.
Thus, the above-mentioned development of the batch cooking technology which mostly took part in the 1980's has been characterized by improvements in terms of energy savings but also provided improved strength delivery of the delignified cellulosic material and made it possible to extend delignification in cooking.
It has, however, been noticed that the introduction of liquor displacement batch systems results in lower turpentine yield. Minor attention has been paid to turpentine recovery as in general; the turpentine recovery has played a minor economical role for mills. In studies, it has however been found that the turpentine is partly found in the pulp discharged from the digester and/or in the spent liquors. Other, non-condensable gases are also influenced by the digester and cooking plant venting and thus turpentine recovery. Thus, other gases may also be found in the discharge pulp and/or in the spent liquors when venting is ineffective. In black liquors, turpentine affects for example the soap solubility and thus changes the behavior of soap. A high turpentine content in black liquors lowers the soap solubility. Soap separation from spent liquors is affected in e.g. the pulp washing area. During the cooking cycle, ineffective removal of turpentine decreases the solubility of extractives, e.g. soap, from the lignocellulosic material into the cooking liquor. The turpentine affects soap in the same way in a pulp suspension and thus higher levels of turpentine cause low solubility of extractives into the liquor phase of a pulp suspension. As a consequence, the pulp is difficult to de-water and wash, and technical problems in washing occur when relieving of turpentine is ineffective. Problems in washing can for example cause production difficulties; increase chemical consumption and lower quality of produced pulp due to higher wash losses in bleach stages. High turpentine levels in the discharge pulp are an environmental harm and safety risks may also occur, as the volatile compounds may evaporate in e.g. the washing plant. As recent studies have shown that high-quality pulp, efficient pulp production and high recovery efficiency of turpentine often work together; development of the liquor-displacement batch system has to occur.
In prior liquor-displacement batch processes, the digester is either degassed to a pressurized spent liquor accumulator wherefrom the gases are vented to the turpentine recovery (e.g. in the RDH system (Foran, C. D., Recovery notes for Kamyr Digester Systems—Cold blow Batch Digester Systems—TMP Process Condensor, Decanter and Storage Systems, 1994 PCA/TAPPI By-Product Recovery Short Course, Mar. 14-16. 1994, Stone Mountain, Ga., p. 17-19)) or the digester is directly vented to the turpentine recovery system (e.g. the cold-blow system (see e.g., Petterson, B., Ernelfeldt, B., “Advances in technology make batch pulping as efficient as continuous”, Pulp & Paper November 1985, p. 90-93)). Combinations of the above-mentioned degassing methods are also found, i.e. both direct degassing of digesters and degassing of accumulators to turpentine recovery. The turpentine recovery itself, i.e. liquor separator, condensers and decanters, does not essentially differ from the one used in conventional batch cooking. When applying degassing from the digester to a pressurized spent liquor accumulator, the accumulator degassing to the turpentine recovery is based on pressure control and the target is to retain overpressure and more particularly a constant overpressure in said accumulator, since the overpressure forces the liquor through heat recovery to an atmospheric tank and suppresses uncontrolled boiling of the liquor. Consequently, little vaporization of volatile compound occurs in the accumulator. The turpentine is solubilized in the black liquor and turpentine recovery will be lower (Foran, C. D., Recovery notes for Kamyr Digester Systems—Cold blow Batch Digester Systems—TMP Process Condensor, Decanter and Storage Systems, 1994 PCA/TAPPI By-Product Recovery Short Course, Mar. 14-16. 1994, Stone Mountain, Ga., p. 18).
Typical of prior liquor displacement processes are also that the digester has a high starting temperature in the actual cooking phase when circulation is applied following chip pretreatment. Accordingly, the digester is heated to the cooking temperature more rapidly than in conventional cooking. Thus, the time at gas-off is short, as no gas-off occurs during chip pretreatment.
Other differences relative to conventional batch cooking are that the digester is operated at a higher liquor-to-wood ratio. Therefore, the turpentine dissolves in the black liquor and the amount of recovered turpentine decreases compared to conventional batch cooking, Methods wherein a portion of hot liquor is removed to create a liquid-vapor interface in the top of the digester followed by removal of the vapors disposed directly to the turpentine recovery have also been suggested, as described in PCT application WO 98/56978 and application 951399. However, our experience of the so-called Cold Blow process using a clear liquid-vapor interface in the top of the digester, liquor circulation to above the liquor-vapor interface and direct degassing to the turpentine recovery, as well as of mill trials using the above-mentioned methods wherein the digester was not hydraulically full, a liquor-gas interface was present and direct degassing was used, also showed that the turpentine yield was not at the level of conventional batch cooking.
Accordingly, a need for an improved liquor-displacement batch process, which more efficiently recovers turpentine and removes other volatile gases more efficiently from the cooking process, is evident.
In continuous cooking processes, the chip material is heated before introduction of the chips into the digester with flash steam obtained from flashing the hot black liquor. The turpentine and non-condensable gases are not removed from the digester during continuous cooking. Instead, the turpentine must be removed from the spent (black) liquor extracted, typically at a temperature of 150-170° C., from the digester. In continuous cooking, the spent liquor is flashed before going to evaporator feed storage. The liquor is flashed in multiple stages, typically twice to a temperature of about 100° C. The primary flash steam is returned to the steaming vessel to preheat the incoming chips. The underflow from the primary flash tank is flashed again. The flash steam from the secondary flash tank in older continuous cooking designs is combined with the gases from the steaming vessel and sent on to a cyclone separator, condensers and turpentine decanter. The primary flash steam contains more turpentine than the secondary flash steam. The drawback of older designs is that the turpentine in the primary flash steam is condensed in the steaming vessel.
In newer designs of continuous digesters, a portion of the secondary flash steam is returned to the bottom of the chip bin to pre-steam the chips. As the secondary flash steam is returned to heat chips in the chip bin, the turpentine in the secondary flash steam condenses on the chips. The heat released from the primary flash steam to heat the chips in the steaming vessel results primarily from the condensation of water. This results in venting of turpentine from the steaming vessel by preventing condensation of primary flash steam turpentine on cold chips in the steaming vessel. In newer continuous cooking designs, the gases from the steaming vessel are sent on to a cyclone separator, condensers and turpentine decanter. Portions of the secondary steam are also conducted to the condensers and turpentine decanter. However, the turpentine recovery yields of continuous cooking is clearly lower than from conventional batch digesters. More details of the turpentine recovery in continuous cooking is found in Foran, C. D., Recovery notes for Kamyr Digester Systems—Cold blow Batch Digester Systems—TMP Process Condensor, Decanter and Storage Systems, 1994 PCA/TAPPI By-Product Recovery Short Course, Mar. 14-16. 1994, Stone Mountain, Ga., p. 4-14. Accordingly, a need for improved recovery of turpentine and other volatile compounds is also evident in continuous cooking.